You need to constantly validate your theories against the real world, what other people say is not enough. This means: solve a lot of problems after you learne

You need to constantly validate your theories against the real world, what other people say is not enough. This means: solve a lot of problems after you learned a theory, try to feel out the problem space yourself by extending the theory or applying it in a specific way, practice, practice, practice, and always look out for things everyone considers true but are not obvious from the data.